Convergent Synthesis of the C1–C29 Framework of Amphidinolide F

The complete carbon framework of the macrocyclic marine natural product amphidinolide F has been prepared by a convergent synthetic route in which three fragments of similar size and complexity have been coupled. Key features of the syntheses of the fragments include the stereoselective construction of the tetrahydrofuran in the C1–C9 fragment by oxonium ylide (free or metal-bound) formation and rearrangement triggered by the direct generation of a rhodium carbenoid from 1-sulfonyl-1,2,3-triazole, the highly diastereoselective aldol reaction between a boron enolate and an aldehyde with 1,4-control to prepare the C10–C17 fragment, and the formation of the tetrahydrofuran in the C18–C29 fragment by intramolecular nucleophilic ring opening of an epoxide with a hydroxyl group under acidic conditions.


■ INTRODUCTION
The cytotoxic marine natural product amphidinolide F was isolated from a dinoflagellate associated with the Okinawan flatworm Amphiscolops magniviridis and its structure reported by Kobayashi and co-workers in 1991 (Figure 1). 1 Amphidinolide F contains a macrolactone that is identical to the core of amphidinolide C, 2 a natural product isolated by the Kobayashi group and reported in 1988, but it bears a truncated side chain (C25−C29). The natural products amphidinolide C2 and C3 share the same macrolactone core structure but, in common with amphidinolide C, have longer and more elaborate side chains than amphidinolide F (Figure 1).
Amphidinolides F, C, C2, and C3 are cytotoxic agents, but amphidinolide C displays significantly higher activity against certain cancer cell lines (e.g., L1210 murine lymphoma and KB epidermoid carcinoma cells) than any of the other three. 3 This observation suggests that the hydroxyl group in the side chain of amphidinolide C confers enhanced cytotoxic activity by either hydrogen bonding or covalent binding to its biological target at this site.
The size, stereochemical complexity, and biological activities of amphidinolides F, C, C2, and C3, have made them attractive targets for total synthesis and stimulated the development of new strategies and synthetic methods that permit rapid access to key subunits found in these natural products. Over the past two decades, substantial portions of all four compounds have been synthesized by the groups of Kobayashi (C1−C10; C17− C29), 4 Armstrong (C18−C29), 5 Carter (C7−C20), 6 Dai (C18−C26), 7 Ferrié(C1−C9), 8 Forsyth (C1−C9; C11− C25; C1−C14; C15−C25), 9 Mohapatra (C1−C9; C19− C34), 10 Morken (C1−C15), 11 Pagenkopf (C1−C9; C18− C34), 12 Roush (C1−C9; C11−C29), 13 Spilling (C1−C9; C18−C29; C18−C34), 14 and Williams (C10−C25). 15 These meticulous and extensive synthetic studies have culminated in the recent total syntheses of amphidinolides F and C by the groups of Furstner and Carter 16,17 and the total syntheses of amphidinolides F and C2 by the group of Ferrie. 18 In previous publications, we have reported the synthesis of the C1−C17 fragment of amphidinolides F, C, C2, and C3 and the C18−C34 fragment of amphidinolides of C, C2, and C3. 19 Our expectation was that the entire carbon skeleton of each natural product would be obtained by the union of two fragments of similar size and complexity through construction of the bond between C17 and C18 ( Figure 1). Although our original strategy was both convergent and logical, we were concerned about the number of steps required to prepare each fragment and the somewhat limited options that would be available for fragment coupling to complete the entire carbon framework. We now report the design and implementation of a convergent second-generation synthetic route to the entire carbon framework of amphidinolide F. The new synthetic route is based on the concise and efficient synthesis of three fragments of similar size and complexity and was designed to provide greater flexibility in the final coupling sequence.
The retrosynthetic analysis of amphidinolide F on which our second-generation synthesis is based is shown in Figure 2. Two primary disconnections by scission of the bond between C9 and C10 (green) or the lactone C−O bond (purple) lead to intermediates (i and ii, respectively) in which the macrocycle has been opened. Further disconnection of i through the ester C−O bond and the C17−C18 bond generates the three key fragments iii, iv, and v. Disconnection of the bond between C9 and C10 and the bond between C17 and C18 in carboxylic acid ii leads to the same fragments (iii−v). This analysis provides flexibility in fragment coupling in the forward direction, with the formation of the macrocycle being accomplished by either a standard macrolactonization reaction or an intramolecular palladium-catalyzed Stille coupling reaction ( Figure 2).

■ RESULTS AND DISCUSSION
Synthetic studies commenced with the construction of the C1−C9 fragment of amphidinolide F. In our previous work, the tandem sequence of copper-catalyzed carbenoid generation, oxonium ylide formation, and rearrangement was used to synthesize an intermediate common to both tetrahydrofuran-containing segments (C1−C7 and C18−C24). 19,20 However, subsequent elaboration of the C1−C7 unit to give the C1−C9 fragment with the required level of stereocontrol at C7 and C8 proved to be rather inefficient. 19a Thus, for the secondgeneration approach, a highly functionalized chiral pool starting material was selected and the pivotal catalytic carbenoid generation, oxonium ylide formation, and rearrangement reaction was modified so that the substituents at C7 and C8 were present prior to construction of the tetrahydrofuran in the C1−C9 fragment.
Synthesis of the C1−C9 fragment of amphidinolide F corresponding to iii in the retrosynthetic analysis ( Figure 2) began with the high-yielding conversion of commercially available tri-O-acetyl-D-glucal (1) into allyl ether 2 by sequential ester cleavage, di-t-butylsilylene protection of the 1,3-diol, and allylation of the remaining hydroxyl group by deprotonation and O-alkylation with allyl bromide (Scheme 1). Acid-mediated hydration of enol ether 2 delivered lactol 3, and the Ramirez olefination procedure was employed to convert this masked aldehyde into 1,1-dibromoalkene 4. 21 Fluoride ion-mediated cleavage of the di-t-butylsilylene protecting group followed by tert-butyldimethylsilyl (TBS) protection of all three hydroxyl groups of the resulting polar triol intermediate provided the fully protected dibromoalkene 5. It was essential to buffer the desilylation reaction with acetic acid to avoid decomposition of the dibromoalkene. Treatment of dibromide 5 with n-butyllithium resulted in sequential metal−halogen exchange, α-elimination, and rearrangement to produce a lithiated terminal acetylene 22 that was reacted immediately with tosyl azide to provide the isomerizationprone 1-sulfonyl-1,2,3-triazole 6, 23 the precursor required for the key carbenoid reaction, which required rapid purification and careful storage.
Triazole 6 was converted into dihydrofuranone 8 by reaction with rhodium(II) acetate (1 mol %) in toluene at reflux and treatment of the intermediate product with basic alumina (Brockmann Grade III) according to the procedure devised by Boyer (Scheme 2). 24 The reaction is presumed to have occurred by rhodium carbenoid generation from the diazo imine formed by Dimroth equilibration of triazole 6, 25 followed by oxonium ylide (free or metal-bound) formation and apparent [2,3]-sigmatropic rearrangement. In situ hydrolysis of the intermediate imine 7 by exposure to basic alumina afforded ketone 8 in a highly diastereoselective manner (d.r. > 20:1). Ketone 8 was then converted into diene 9 by a Peterson olefination procedure in which the ketone was reacted with the organocerium reagent generated from (trimethylsilyl)methylmagnesium bromide, and the resulting hydroxysilane was treated with sodium bis(trimethylsilyl)amide to effect elimination. 26 It was necessary to use an organocerium reagent with reduced basicity to avoid epimerization at the site adjacent to the carbonyl group (C3). Selective dihydroxylation of the terminal alkene under standard Upjohn conditions produced 1,2-diol 10 as an inconsequential diastereomeric mixture. Subsequent periodate cleavage of the diol and reduction of the resulting aldehyde provided alcohol 11.
Conversion of alcohol 11 into the fully elaborated C1−C9 fragment was accomplished by the reaction sequence shown in Scheme 3. Attempted stereocontrolled conversion of the exocyclic alkene of alcohol 11 into the C4 methyl substituent by hydrogenation in the presence of Crabtree's catalyst was unsuccessful. 27 In contrast, rapid and highly diastereoselective directed hydrogenation of the alkene was accomplished when an NHC−iridium(I) complex developed by Kerr and coworkers was employed as the catalyst. 28 Immediate acylation of the hydroxyl group with pivaloyl chloride afforded ester 12; hydrogenation and esterification reactions could be performed in a one-pot fashion. Subsequent selective cleavage of a single TBS ether to give primary alcohol 13 was accomplished in good yield by treatment of ester 12 with the hydrogen fluoride pyridine complex at 0°C. Oxidation of the alcohol to give the corresponding aldehyde 14 was performed by the use of the Dess−Martin protocol, and alkyne 15 was obtained by the use of the Ohira−Bestmann modification 29 of the Seyferth− Gilbert homologation reaction. 30 The final step required to complete the C1−C9 fragment was the conversion of alkyne 15 into vinylic stannane 16. The alkyne hydrostannation protocol developed by Kazmaier and co-workers proved to be uniquely effective for this transformation. 31 Thus, the treatment of alkyne 15 with tri-n-butyltin hydride and a substoichiometric amount (10 mol %) of Mo(CO) 3 (t-BuNC) 3 , along with butylated hydroxytoluene (BHT) as a radical inhibitor, at 55°C in tetrahydrofuran (THF) afforded the required vinylic stannane 16 in a 69% yield as well as a small quantity (16% yield) of the regioisomeric E-alkenyl stannane. The isomeric stannanes were readily separable by chromatography on silica gel.
The synthesis of the C10−C17 fragment corresponding to iv in the retrosynthetic analysis ( Figure 2) commenced with a diastereoselective aldol reaction between a boron enolate derived from the known methyl ketone 17 32 and aldehyde 18, an intermediate that we had used in previous studies concerning the synthesis of the amphidinolides (Scheme 4). 19a Thus, the treatment of ketone 17 with dicyclohexylboron chloride and triethylamine produced a boron enolate and subsequent aldol reaction with aldehyde 18 produced βhydroxyketone 19 in an 81% yield and with >20:1 diastereoselectivity. This result is consistent with the findings of Paterson and co-workers who have reported highly syn-selective 1,4-stereoinduction during aldol reactions of enolates generated from the benzyl ether analogue of ketone 17 with aldehydes 33 and have used a closely related aldol reaction in their synthesis of the marine natural product phorbaside A. 34 The stereochemical outcome of the reaction and the high level of diastereocontrol can be explained by invoking the model proposed by Paton and Goodman to account for the stereochemical outcome of aldol reactions between aldehydes and boron enolates derived from analogous ketones. 35 β-Hydroxyketone 19 was then subjected to a highly diastereoselective Evans−Tishchenko reduction reaction with pivaldehyde to produce alcohol 20. 36 TBS protection of the free hydroxyl group and hydrogenolytic cleavage of the PMB ether afforded alcohol 21. Oxidation of the alcohol to give aldehyde 22 was followed by Seyferth−Gilbert homologation according to the Ohira−Bestmann protocol. 29 The resulting alkyne 23 was converted into alkenylsilane 24 by silylcupration and reaction of the resulting organocopper intermediate with methyl iodide. 37 Treatment of silane 24 with N-iodosuccinimide resulted in the stereoretentive replacement of the silyl group with iodine. Selective fluoride-mediated removal of the TBS group to give a free primary hydroxyl group delivered iodide 25 required for the subsequent Stille coupling to the C1−C9 fragment 16.
Two fragments corresponding to the C18−C29 unit of amphidinolide F were prepared so that two distinct coupling strategies for construction of the C17−C18 bond could be explored. In previous studies, the tetrahydrofuran had been constructed by the intramolecular reaction of a metal carbenoid with an allyl ether, but for the purposes of the second-generation approach, alternative ring construction methods were explored.
The syntheses of both C18−C29 variants commenced with the known epoxide 26, which can be prepared from D-aspartic acid in three steps. 38 Epoxide 26 was subjected to nucleophilic ring opening by reaction with propargylmagnesium bromide in the presence of a substoichiometric amount (1 mol %) of mercury(II) chloride to give alcohol 27, 39 which was TBSprotected to give alkyne 28 (Scheme 5). Deprotonation of alkyne 28 with n-butyllithium and reaction of the resulting anion with formaldehyde afforded propargylic alcohol 29. Lindlar reduction of the alkyne delivered the Z-allylic alcohol 30, and subsequent Sharpless asymmetric epoxidation, with (−)-diethyl D-tartrate as the ligand, 40 produced epoxide 31 (d.r. 9:1). Swern oxidation of the alcohol produced aldehyde 32, and the Ohira−Bestmann protocol was employed immediately to convert this compound into alkyne 33, 29 the cyclization precursor.
Construction of the tetrahydrofuran-containing C18−C29 fragment from epoxide 33 was now investigated (Scheme 6). Fluoride ion-mediated removal of both TBS groups and treatment of the resulting epoxy diol with camphorsulfonic acid in dichloromethane at −40°C resulted in regioselective intramolecular nucleophilic opening of the epoxide by the secondary alcohol to give the known tetrahydrofuran 34, 7 the structure of which was confirmed by comparison of NMR data with that in the literature and by its conversion into the primary t-butyldiphenylsilyl ether that had been prepared in our previous studies on the synthesis of amphidinolides C, C2, and C3. 19b Immediate acylation of the primary hydroxyl group with pivaloyl chloride then provided the propargylic alcohol 35 in a 75% yield over three steps. A copper-free Sonogashira coupling reaction 41 was then used to couple the alkyne to 1bromo-2-methyl-1-propene, and the subsequent TBS protection of the hydroxyl group delivered enyne 36. Reductive cleavage of the pivaloyl ester, by treatment with lithium aluminum hydride, provided primary alcohol 37, and this compound was converted into aldehyde 38 by oxidation with the Dess−Martin periodinane. The aldehyde was then treated with 1,3-propanedithiol under Lewis acidic conditions to give dithiane 39. Sequential cleavage of the TBS ether, stereoselective partial alkyne reduction by treatment with Red-Al to deliver the E-allylic alcohol in a highly stereoselective manner, and reprotection of the free hydroxyl group as a triethylsilyl (TES) ether afforded diene 40, corresponding to C18−C29 of the natural product, in a 47% yield over four steps. This fragment was now ready for coupling to the C1−C17 unit.
The second C18−C29 fragment was prepared from alkyne 34 by a significantly shorter route than that shown in Scheme 6. Sonogashira coupling of the terminal alkyne to 1-bromo-2methyl-1-propene afforded enyne 41 (Scheme 7). The propargylic alcohol was then subjected to reduction with Red-Al to deliver E-allylic alcohol 42, and both hydroxyl groups were TES-protected to give diene 43 with an overall yield of 62% over three steps. This diene had been prepared by Kobayashi and co-workers during the synthetic work performed to establish the configuration of stereogenic centers in amphidinolide C, and the data for our sample match those reported in the literature. 4 Selective cleavage of the primary TES ether to produce alcohol 44 and subsequent oxidation with the Dess−Martin periodinane afforded aldehyde 45. It should be noted that this aldehyde is the direct TES ether analogue of intermediates prepared by the groups of Armstrong and Ferriéduring their studies on the synthesis of amphidinolide F. 5,18 Direct formation of dithiane 40 by the Lewis acid mediated reaction of aldehyde 45 with 1,3propanedithiol was attempted to shorten the sequence in Scheme 6, but decomposition of aldehyde 45 was observed.
The full carbon framework of amphidinolide F was now assembled by coupling of the C1−C9, C10−C17, and C18− C29 fragments (Scheme 8). Alkenyl iodide 25 was first coupled to the vinylic stannane 16 under modified Stille conditions to give the C1−C17 fragment in an 82% yield. 42 The hydroxyl group at C17 was replaced with iodine under standard iodination conditions to give the iodide 46 in 93% yield. Deprotonation of dithiane 40 with t-butyllithium in THF-hexamethylphosphoramide (HMPA) and reaction of the resulting anion with iodide 46 afforded the fully coupled product 47, a compound that corresponds to the entire C1− C29 framework of amphidinolide F. However, the coupled product was obtained in only 13% yield and significant amounts of both dithiane 40 (42%) and iodide 46 (51%) were recovered from the reaction. Attempts to improve the yield of this coupling reaction were not successful, and the product yield was deemed to be unacceptably low.
To address the issue of incomplete reaction and the resulting low yield obtained when coupling iodide 46 to dithiane 40, reversal of the polarity of the fragments during the C−C bondforming reaction was investigated. In this case, aldehyde 45 was reacted with an organolithium reagent generated from iodide 46. Thus, treatment of iodide 46 with t-butyllithium to effect the metal−halogen exchange and addition of the resulting organolithium intermediate to aldehyde 45 was expected to deliver a diastereomeric mixture of alcohols 48. Unfortunately, treatment of iodide 46 with t-butyllithium followed by immediate addition of aldehyde 45 resulted in decomposition of the iodide instead of formation of the required alcohol 48. Addition of t-butyllithium to a mixture of iodide 46 and aldehyde 45 in diethyl ether at −78°C also failed to deliver the required alcohol 48.
In summary, the entire carbon framework of amphidinolide F has been assembled by the union of three fragments: stannane 16 (C1−C9), iodide 25 (C10−C17), and dithiane 40 (C18−29). The synthesis of each fragment has been accomplished in a highly stereocontrolled manner. In the case of the C1−C9 fragment, oxonium ylide (free or metal-bound) formation and rearrangement initiated by the generation of a rhodium carbenoid from a 1-sulfonyl-1,2,3-triazole has been used to assemble the tetrahydrofuran with a high level of diastereocontrol at the C3 stereogenic center. The C10−C17 fragment has been assembled by a stereoselective aldol reaction of a boron enolate with 1,4-diastereocontrol followed by an Evans−Tishchenko reduction reaction of the resulting βhydroxyketone. Efficient construction of the tetrahydrofuran in the C18−C29 fragment has been accomplished by acidpromoted intramolecular 5-exo nucleophilic ring opening of an epoxide with a hydroxyl group. Further synthetic studies are required to optimize fragment coupling and complete the synthesis of amphidinolide F. The results of this work will be reported in due course.

■ EXPERIMENTAL SECTION
Reagents were purchased from commercial suppliers and used without purification unless otherwise stated. Air-and moisturesensitive reactions were performed under an atmosphere of argon in a flame-dried apparatus. Tetrahydrofuran, toluene, acetonitrile, dichloromethane, and diethyl ether were purified using a Pure-SolvTM 500 Solvent Purification System. Petroleum ether used for chromatography was the 40−60°C fraction. All reactions were monitored by thin-layer chromatography (TLC) using Merck silica gel 60 covered aluminum backed plates F254. TLC plates were visualized under UV light and stained using potassium permanganate solution, acidic ethanolic anisaldehyde solution, or phosphomolybdic acid solution. Flash column chromatography was performed with silica gel (Fluorochem LC60A 35−70 μm or Geduran Si 60 35−70 μm) as solid support. IR spectra were recorded using a Shimadzu FT IR-8400S ATR instrument. The IR spectrum of each compound (solid or liquid) was acquired directly on a thin layer at ambient temperature. 1 H NMR spectra were recorded on Bruker Avance III 400 and 500 MHz spectrometers at ambient temperature. 13 C NMR spectra were recorded on Bruker Avance III 400 and 500 MHz spectrometers at 101 and 126 MHz at ambient temperature, respectively. Optical rotations were recorded using an Autopol V polarimeter. High-and low-resolution mass spectra (HRMS) were performed by the use of positive chemical ionization or electron impact ionization on a Jeol MStation JMS-700 instrument or by the use of positive or negative ion electrospray techniques on a Bruker micrOTOF-Q instrument. Elemental analyses were carried out on an Exeter Analytical Elemental Analyser EA 440. Melting points were recorded with an Electrothermal IA 9100 apparatus.
To a solution of the crude dithiane 39 (303 mg) in THF (7 mL) at 0°C was added tetra-n-butylammonium fluoride (1.4 mL of a 1.0 M solution in THF, 1.4 mmol) dropwise. The resulting solution was stirred at rt for 1 h, and then water (7 mL) was added. The phases were separated, and the aqueous phase was extracted with diethyl ether (3 × 7 mL). The combined organic extracts were washed with brine (20 mL), dried (anhydrous MgSO 4 ), filtered, and concentrated. The residue was used directly in the next step without purification.
To a solution of the crude propargylic alcohol (155 mg) in THF (10 mL) at 0°C was added sodium bis(2-methoxyethoxy)aluminum hydride (670 μL of a ≥65 wt % in toluene, 2.2 mmol) dropwise. The resulting cloudy mixture was stirred at rt for 30 min and then cooled to 0°C, and saturated aqueous potassium sodium tartrate (20 mL) was added. The phases were separated, and the aqueous phase was extracted with diethyl ether (3 × 20 mL). The combined organic extracts were washed with brine (50 mL), dried (anhydrous MgSO 4 ), filtered, and concentrated. The residue was used directly in the next step without purification.
To a solution of crude allylic alcohol (156 mg) in dichloromethane (10 mL) at −78°C were added 2,6-lutidine (0.17 mL, 1.5 mmol) and triethylsilyl trifluoromethanesulfonate (0.17 mL, 0.74 mmol) sequentially. The resulting solution was stirred at −78°C for 30 min, and then water (8 mL) was added. The biphasic mixture was allowed to warm to rt, and the phases were separated. The aqueous phase was extracted with dichloromethane (3 × 8 mL), and the resulting mixture was stirred for 15 h. The reaction was quenched by the dropwise addition of saturated aqueous sodium bicarbonate (150 mL). The biphasic mixture was allowed to warm to rt, and the phases were separated. The aqueous phase was extracted with diethyl ether (3 × 150 mL), and the combined organic extracts were washed with brine (400 mL), dried (anhydrous MgSO 4 ), filtered, and concentrated. The residue was purified by flash chromatography on silica gel (pet. ether-ethyl acetate, 85:15) to afford alcohol 44 (130 mg, 79%) as a colorless oil. R f = 0.21 (pet. ether-ethyl acetate, 85:15); [α] D 24